Ceramic matrix composites having monomodal pore size distribution and low fiber volume fraction

ABSTRACT

Ceramic matrix composite articles include, for example, a plurality of unidirectional arrays of fiber tows in a matrix having a monomodal pore size distribution, and a fiber volume fraction between about 15 percent and about 35 percent. The articles may be formed by, for example, providing a shaped preform comprising a prepreg tape layup of unidirectional arrays of fiber tows, a matrix precursor, and a pore former, curing the shaped preform to pyrolyze the matrix precursor and burnout the pore former so that the shaped preform comprises the unidirectional arrays of fiber tows and a porous matrix having a monomodal pore size distribution, and subjecting the cured shaped preform to chemical vapor infiltration to densify the porous matrix so that the ceramic matrix composite article has a fiber volume fraction between about 15 percent and about 35 percent.

TECHNICAL FIELD

The present disclosure generally relates to ceramic matrix composites,and more particularly, to articles and methods for forming ceramicmatrix composite articles having monomodal pore size distribution, andoptimized fiber volume fraction.

BACKGROUND

Ceramic matrix composites (CMCs) generally include a ceramic fiberreinforcement material embedded in a ceramic matrix material. Thereinforcement material serves as the load-bearing constituent of the CMCin the event of a matrix crack, while the ceramic matrix protects thereinforcement material, maintains the orientation of its fibers, andserves to dissipate loads to the reinforcement material. Of particularinterest to high-temperature applications, such as in gas turbines, aresilicon-based composites, which include silicon carbide (SiC) as thematrix and/or reinforcement material.

Different processing methods have been employed in forming CMCs. Forexample, one approach includes chemical vapor infiltration (CVI). CVI isa process whereby a matrix material is infiltrated into a fibrouspreform by the use of reactive gases at elevated temperature to form thefiber-reinforced composite. For example, conventional cloth based CMCsformed by CVI typically have a porosity between 10 percent and 20percent, a fiber volume fraction between 35 percent and 40 percent, andan interlaminar tensile (ILT) strength between 1 ksi and 3 ksi, asmeasured by a standard 1 inch diameter button pull test. CVI compositematrices typically have no free silicon phase, and thus have good creepresistance and the potential to operate at temperatures above 2,570degrees Fahrenheit.

Another approach includes melt infiltration (MI), which employs moltensilicon to infiltrate into a fiber-containing preform. For example,conventional unidirectional tape-based CMCs formed by MI typically havea porosity of below 3 percent, a fiber volume fraction between 20percent and 33 percent, and an interlaminar tensile (ILT) strengthbetween 5 ksi and 9 ksi. The matrix of MI composites contains a freesilicon phase (i.e. elemental silicon or silicon alloy) that limits useof the CMC to below that of the melting point of the silicon or siliconalloy, or about 2,550 degrees Fahrenheit to 2,570 degrees Fahrenheit.Moreover, the free silicon phase causes the MI SiC matrix to haverelatively poor creep resistance.

Another approach employs a partial CVI process followed by an MIprocess, and is generally referred to as “slurry cast MI.” This approachusually yields an intermediate porosity between that of MI compositesand CVI composites, generally of about 6 percent, a fiber volumefraction between 35 percent and 40 percent, an interlaminar tensile(ILT) strength between 2 ksi and 4 ksi, and also contains residual freesilicon phase within the composite matrix.

SUMMARY

Shortcomings of the prior art are overcome and additional advantages areprovided through the provision, in one embodiment, of a method forforming a ceramic matrix composite article. The method includes, forexample, providing a shaped preform comprising a prepreg tape layup ofunidirectional arrays of fiber tows, a matrix precursor, and a poreformer, curing the shaped preform to pyrolyze the matrix precursor andburnout the pore former so that the shaped preform comprises theunidirectional arrays of fiber tows and a porous matrix skeleton havinga monomodal pore size distribution, and subjecting the cured shapedpreform to chemical vapor infiltration to densify the porous matrixskeleton so that the ceramic matrix composite article has a fiber volumefraction between about 15 percent and about 35 percent.

In another embodiment, a method for forming a ceramic matrix compositearticle includes, for example, providing a shaped preform comprising aprepreg tape layup of unidirectional arrays of fiber tows, a matrixprecursor for forming a ceramic matrix, a particulate filler, and a poreformer, curing the shaped preform to pyrolyze the matrix precursor andburnout the pore former so that the shaped preform comprises theunidirectional arrays of fiber tows and a porous ceramic matrix skeletonhaving a monomodal pore size distribution with a median pore size ofbetween about 1 micrometers and about 30 micrometers, and subjecting thecured shaped preform to chemical vapor infiltration with gaseousceramic, a partial chemical vapor infiltration and melt infiltration, ora partial chemical vapor infiltration, slurry casting, and meltinfiltration, to densify the porous ceramic matrix skeleton so that theceramic matrix composite article has a fiber volume fraction betweenabout 15 percent and about 35 percent.

In another embodiment, a ceramic matrix composite article includes, forexample, a plurality of unidirectional arrays of fiber tows in a matrixhaving a monomodal pore size distribution, and wherein the ceramicmatrix composite article comprises a fiber volume fraction between about15 percent and about 35 percent.

DRAWINGS

The foregoing and other features, aspects and advantages of thisdisclosure will become apparent from the following detailed descriptionof the various aspects of the disclosure taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a cross-sectional view of a ceramic matrix composite articleaccording to an embodiment of the present disclosure;

FIG. 2 is a flowchart of a method according to an embodiment of thepresent disclosure for forming the ceramic matrix composite article ofFIG. 1;

FIG. 3 is a cross-sectional view of an uncured preform having aplurality of unidirectional prepreg tapes for use in the forming of theceramic matrix composite article of FIG. 1;

FIG. 4 is a cross-sectional view of a cured preform formed from theuncured preform of FIG. 3;

FIG. 5 shows a schematic representation of a conventional CVI preformmade from a woven fiber tow;

FIG. 6 is an idealized representation of the pore size distribution of apreform and final CVI-densified composite article formed according tothe present disclosure compared to a preform and CVI-densified compositeformed using a woven fiber layup as is typically used for conventionalCVI;

FIG. 7 is flowchart of a method according to one embodiment of thepresent disclosure for forming a ceramic matrix composite article;

FIG. 8 is flowchart of a method according to one embodiment of thepresent disclosure for forming a ceramic matrix composite article; and

FIG. 9 is flowchart of a method according to one embodiment of thepresent disclosure for forming a ceramic matrix composite article.

DETAILED DESCRIPTION

Embodiments of the present disclosure and certain features, advantages,and details thereof, are explained more fully below with reference tothe non-limiting examples illustrated in the accompanying drawings.Descriptions of well-known materials, processing techniques, etc., areomitted so as not to unnecessarily obscure the disclosure in detail. Itshould be understood, however, that the detailed description and thespecific examples, while indicating embodiments of the presentdisclosure, are given by way of illustration only, and not by way oflimitation. Various substitutions, modifications, additions, and/orarrangements, within the spirit and/or scope of the underlying inventiveconcepts will be apparent to those skilled in the art from thisdisclosure.

FIG. 1 illustrates a portion of a ceramic matrix composite (CMC) article10 according to an embodiment of the present disclosure. CMC article 10may include a ceramic fiber reinforcement material in a ceramic matrixmaterial. As described in greater detail below, in some embodiments, CMCarticle 10 may be formed by a process resulting in CMC article 10 havinga plurality of unidirectional arrays of fiber tows 20, and a densifiedmatrix 30. Such a CMC article may be tailored to have improvedproperties such as, but not limited to, mechanical properties (e.g.,interlaminar (ITL) strength and proportional limit (PL)), and oxidationresistance.

As further described below, for example, pre-coated fiber tows,prepregging, ply layup, consolidation, and burnout may result in a curedpreform for subsequent densification. A slurry may be used duringprepregging, having for example a matrix precursor along withparticulate fillers and pore formers such as polymeric pore formers toadjust the fiber spacing and pore size distribution and give afree-standing preform for CVI densification. After curing of thepreform, e.g., pyrolysis of the matrix precursor and burnout of the poreformers, the cured preform can be densified using CVI alone, using acombination of a partial CVI followed by melt infiltration with silicon,silicon alloy or an oxide, such as rare-earth disilicates (RE₂Si₂O₇), orusing slurry infiltration prior to melt infiltration. Advantages ofusing tow-based unidirectional ply preforms may give more uniform porestructure for densification resulting in a more uniform CMCmicrostructure. Touching of fibers and continuous coatings may beeliminated, thereby improving mechanical properties and oxidationresistance of a CMC article. Such a technique of the present disclosuremay be advantageous for application to silicon-bearing ceramic turbinecomponents, for example, turbine blades, vanes, nozzles, shrouds,combustors, etc., and repairs thereof.

FIG. 2 illustrates a method 100 for forming ceramic matrix compositearticle 10 (FIG. 1) in accordance with an embodiment of the presentdisclosure. In this exemplary embodiment, method 100 generally includes,at 110, providing a shaped preform comprising a prepreg tape layup ofunidirectional arrays of fiber tows, a matrix precursor, and a poreformer, at 120 curing the shaped preform to pyrolyze the matrixprecursor and burnout the pore former so that the shaped preformcomprises the unidirectional arrays of fiber tows and a porous matrixskeleton having a monomodal pore size distribution, and at 130subjecting the cured shaped preform to chemical vapor infiltration todensify the porous matrix skeleton so that the ceramic matrix compositearticle has a fiber volume fraction between about 15 percent and about35 percent.

FIG. 3 illustrates an uncured shaped preform 200 fabricated from aplurality of prepreg layers 210 in the form of tape-like structuresunidirectionally-aligned tows impregnated with a slurry 214 to create agenerally two-dimensional laminate. The prepreg may be formed form, forexample, a reinforcement material of a desired CMC and a slurry, whichslurry may include a matrix precursor, a pore formers, particulatefillers, and a carrier, as described below. The slurry can be rollmilled to deagglomerate and disperse the powders. The slurry can beinfiltrated into the coated tows by passing the tows through a bath ofthe slurry. The tow can then be wound onto a drum and may includepartial drying of the slurry such that a tape is formed. The tape can beremoved from the drum and unidirectional preform plies can be cut toform the tape.

Materials for the tows may include silicon carbide (SiC) fibers,polycrystalline SiC fibers, or other suitable fiber. An example of amaterial suitable for the tows is HI-NICALON® from NGS Advanced FibersCo. LTD. A suitable range for the diameters of the fibers is about fiveto about twenty micrometers, though fibers with larger and smallerdiameters are also within the scope of this disclosure. The fibers maybe preferably coated with materials such as a carbon or boron nitrideinterface layer (not shown) to impart certain desired properties to theCMC article, e.g., allows slippage between coating and the formed matrixmaterial of the CMC article. A fiber tow, for example, may be a singlebundle of about 500 individual fibers.

The slurry may include a matrix precursor such as organic or inorganicmaterial that leaves char/residue after burnout such as pyrolysis orfiring. In some embodiments, the matrix precursor may include a siliconcontaining precursor operable, as described below, for forming a poroussilicon containing precursor such as silicon carbide in the curedpreform. Examples of a matrix precursor include tetraethyl orthosilicate(TEOS), polycarbosilanes, polysilazanes, polysiloxanes, phenolics, andfuranic compounds. A pore former may include a particle or other speciesthat can remain present through a consolidation process but can befugitive in the burnout or pyrolysis process resulting in a pore.Examples of a pore former can comprise polyethylene, polypropylene,polyamide, nylon, polytetrafluoroethylene (PTFE), polystyrene, polyvinylacetate, polyvinyl alcohol, and/or cellulosic powders. Fillers mayinclude an oxide or non-oxide particle or whisker that helps controlshrinkage. Examples of a filler include SiC, B₄C, SiO₂, HfC, HfB₂, ZrC,ZrB₂, MoSi₂, Si₃N₄, Al₂O₃, rare earth silicates, and rare earthsilicides. A carrier may include organic or inorganic liquid thatdissolves or carries the matrix precursor and other ingredients.Examples of a carrier include water, isopropanol, toluene, and acetone.

The particles included in the pore former may include a monomodalparticle size distribution for a collection of particles which have asingle clearly discernable maxima on a particle size distribution curveas compared to a collection of particles having a bimodal particle sizedistribution having two clearly discernable maxima on a particle sizedistribution curve, or a for a collection of particles having amultimodal particle size distribution of three or more clearlydiscernable maxima on a particle size distribution curve. The particlesincluded in a pore former may include a median size in the range of fromabout 1 micrometer to about 30 micrometers, may include a median size inthe range of from about 1 micrometer to about 20 micrometers, mayinclude a median size in the range of from about 3 micrometer to about10 micrometers, and/or may include a median size in the range of fromabout 3.5 micrometers to about 8 micrometers. Prior to passing the towsthrough a bath of the slurry, the slurry including the matrix precursor,the pore formers, the particulate fillers, and the carrier may becombined and mixed until a uniform mixture is obtained with the poreformers having a uniform spacial distribution.

The plurality of plies of the resulting prepregs are laid-up or stackedinto a desired pattern and shape, and typically arranged so that tows ofthe prepreg layers are oriented parallel, transverse (e.g.,perpendicular), or at an angle relative to other tows of the prepreglayers in other plies. The plurality of layers may typically undergoconsolidation or debulking while subjected to applied pressure and anelevated temperature, such as in a vacuum or in and autoclave orlocalized application of pressure and heat.

The consolidated plurality of stacked plies is subjected to burnout suchas pyrolysis or heated in vacuum or in an inert or a reactive atmospherein order to decompose the matrix precursor, to form a ceramic or ceramicchar, and where the pore former is, for example, volatilized, andproduces a porous preform for chemical vapor infiltration, resulting incured preform 300 illustrated in FIG. 4. The resulting porosity ofprecursor matrix may have a predominantly monomodal pore sizedistribution and predominantly uniform spacial distribution. Forexample, the local maxima in the pore size distribution of the curedporous silicon-containing precursor may be between about 1 micrometer toabout 30 micrometers, about 1 micrometers to about 20 micrometers, about3 micrometer to about 10 micrometers, and/or about 3.5 micrometers toabout 8 micrometers. The cured preform may have a volume porosity ofabout 35 percent to about 65 percent.

The cured preform is then subject to chemical vapor infiltration, suchas with a gaseous source of silicon carbide supplied externally. Thegaseous silicon carbide source infiltrates into the porosity, reacts todeposit SiC on the internal pore surfaces of the porous layer to form adensified silicon carbide matrix of CMC article 10 as shown in FIG. 1,and may contain no free Si metal. An appropriate chemical vaporinfiltration gas may include methyl-trichlorosilane,dimethyl-dichlorosilane, silane+methane, tetrachlorosilane+methane, andother suitable gases.

The resulting porosity of CMC article 10 may have a monomodal pore sizedistribution. For example, the median pore size of the CVI-densified CMCarticle may be about 1 micrometers to about 20 micrometers, or about 1micrometers to about 15 micrometers. CMC article 10 may have a volumeporosity of about 5 percent to about 20 percent. The CMC article mayhave a uniform spacial distributed fiber volume percentage. For example,the CMC article may have a fiber volume of between about 15 percent andabout 35 percent. In other embodiments, a CMC article may be tailored tohave different fiber volume throughout the CMC based on the layup andtape prepregs. For example, CMC article may include at least one firstportion having a first fiber volume percentage and at least one secondportion having a second fiber volume percentage different from saidfirst fiber volume percentage.

Those skilled in the art will appreciate that the teachings of thisdisclosure are also applicable to other CMC material combinations, andthat such combinations are within the scope of this disclosure. Suitablematerials for use in the chemical vapor infiltration process may includesilicon carbide, silicon nitride, silicon oxy-nitride, siliconoxy-carbide, silicon dioxide, aluminum nitride, aluminum oxide, boroncarbide, zirconium carbide, hafnium carbide, zirconium diboride, hafniumdiboride, molybdenum silicides, and other suitable material.

Testing of CMC articles formed in accordance with the technique of thepresent disclosure, including predominantly monomodal pore sizedistribution, showed interlaminar tensile (ILT) strength values of about6 ksi to about 12 ksi for CMCs with 0/90 architecture and a fiber volumefraction of about 18 percent ([0:90]2 s architecture, 0.1″ thick) to 28percent ([0:90]2 s architecture, 0.065″ thick), which are significantlyhigher than the ILT values for conventional CVI composites made fromwoven fibers, and are comparable to, or better than, typical values forMI-type ceramic composites.

FIG. 5 shows a schematic representation of the microstructure of aconventional CVI composite preform made with woven fibers. Thecross-over of the fiber tows in the weave pattern tend to compress thetows into tight bundles. Also, due to the surface roughness of the wovenfiber cloth, the fiber plies tend to pack inefficiently. Themicrostructure of the conventional CVI preform thus has two distincttypes of porosity; the first being the small inter-fiber pores withinthe fiber tows, and the second being the larger inter-tow pores causedby the weaving pattern and the dis-registry of this pattern at the plyboundaries.

FIG. 6 shows an idealized representation of the pore size distributionsfor CVI preforms and final densified composites made using theconventional woven fiber-based CVI approach and by the technique of thepresent disclosure. The two populations of pores illustrated in FIG. 5and described in the previous paragraph lead to a bimodal or multimodalpores size distribution for the conventional woven fiber CVI preforms.By using the process outlined in the current invention, and described inFIGS. 7-9, a preform microstructure as shown in FIG. 1 is obtained,which has a monomodal pore size distribution. Following densificationvia CVI or a combination of CVI and MI processes, the amount of porosityis reduced and the means of the peaks in the pore size distributions maybe shifted, but the multimodal or monomodal nature of the distributionsis retained. It is the larger pores, e.g. above 30 micrometers in size,that are primarily responsible for limiting the interlaminar tensilestrength and proportional limit strength of conventional cloth-based CVIcomposites. Composites made by the present disclosure eliminate, orminimize, the amount of this undesirable large porosity, resulting inthe improved interlaminar tensile strengths cited.

The present inventors' work indicates that, for a specimen of constantthickness, the interlaminar tensile (ILT) strength is inversely relatedto the fiber volume fraction, as long as the fibers remain homogenouslydispersed within the matrix and as long as the porosity remainspredominantly monomodal. On the other hand, the ultimate tensilestrength (UTS) and the proportional limit (PL) are directly related tothe fiber volume fraction.

Therefore, an optimum balance of properties for a specific applicationmay include CMC articles in accordance with the present disclosurehaving fiber volumes of about 15 percent to about 35 percent compared tofiber volumes of 35 percent to 40 percent normally used for conventionalCVI composites. In some embodiments as noted above, portions of aceramic matrix composite article may have different of fiber volumepercentages based on the desired properties of the different portions ofthe ceramic matrix composite article. For example, some ceramic matrixcomposite articles may have portions or regions that have a lower fibervolume percentage compared to other portions or regions that have ahigher fiber volume percentage.

FIG. 7 illustrates a method 500 for forming ceramic matrix compositearticles in accordance with an embodiment of the present disclosure. Inthis exemplary embodiment, method 500 generally includes, at 510 coatingfiber tows, at 520 prepregging the tows to form prepreg tape, and at 530cutting the prepreg tape and laying up an uncured preform for formingthe article. At 540, the preform is consolidated such as in an autoclaveunder heat and pressure. At 550, the preform is subject to a burn-outprocess so that, for example, the resulting preform has a monomodal poresize distribution. At 560, the cured perform is subjected to chemicalvapor infiltration to densify the cured preform to form a finishedceramic matrix composite articles at 570. A ceramic matrix compositearticles formed by method 500 may have an optimized range ofinterlaminar (ILT) strength and proportional limit (PL) with a fibervolume of between about 15 percent and 35 percent, and a volume porosityof about 8 percent to about 20 percent. The ceramic matrix of theceramic matrix composite may have a monomodal pore size distributionwith a median pore size of about 3 micrometer to about 30 micrometers.The ceramic matrix of the ceramic matrix composite may have a uniformspacial pore distribution. Such a ceramic matrix composite article maybe advantageous for application to silicon-bearing ceramic turbinecomponents, for example, turbine blades, vanes, nozzles, shrouds,combustors, etc., and repairs thereof.

In the chemical vapor infiltration (CVI) process, a matrix material suchas silicon carbide is infiltrated into a fibrous preform by the use ofreactive gases at elevated temperature. Generally, limitationsintroduced by having reactants diffuse into the preform and by-productgases diffusing out of the perform result in relatively high residualporosity of between about 12 percent and about 15 percent in thecomposite. In the forming of the CMCs using CVI, the inner portion ofthe composite formed by CVI typically has a higher porosity than theporosity of the outer portion. The CVI composite matrices typically haveno free silicon phase, good creep resistance and the potential tooperate at temperatures above 2,570 degrees Fahrenheit.

FIG. 8 illustrates a method 600 for forming ceramic matrix compositearticles in accordance with an embodiment of the present disclosure. Inthis exemplary embodiment, method 600 generally includes, at 610 coatingfiber tows, at 620 prepregging the tows to form prepreg tape, and at 630cutting the prepreg tape and laying up an uncured preform for formingthe article. At 640, the preform is consolidated such as in an autoclaveunder heat and pressure. At 650, the preform is subject to a burn-outprocess so that, for example, the preform matrix has a monomodal poresize distribution. At 660, the cured perform is subjected to a chemicalvapor infiltration to densify the cured preform resulting in a volumeporosity of about 12 percent to about 35 percent. Further densificationmay occur at 665 with melt infiltration process to form a finishedceramic matrix composite articles at 570. The melt infiltration mayinclude silicon, silicon alloy, a silicide, an oxide, or a combinationthereof. In method 600, the step of chemical vapor infiltration may be apartial or full chemical vapor infiltration compared to the chemicalvapor infiltration process of method 500 (FIG. 6). A ceramic matrixcomposite articles formed by method 600 may have a volume porosity ofless than about 5 percent. The ceramic matrix of the ceramic matrixcomposite may have a monomodal pore size distribution with a median poresize of about 1 micrometers to about 20 micrometers. The ceramic matrixof the ceramic matrix composite may have a uniform spacial poredistribution. Such a ceramic matrix composite article may beadvantageous for application to silicon-bearing ceramic turbinecomponents, for example, turbine blades, vanes, nozzles, shrouds,combustors, etc., and repairs thereof.

FIG. 9 illustrates a method 700 for forming ceramic matrix compositearticles in accordance with an embodiment of the present disclosure. Inthis exemplary embodiment, method 700 generally includes, at 710 coatingfiber tows, at 720 prepregging the tows to form prepreg tape, and at 730cutting the prepreg tape and laying up an uncured preform for formingthe article. At 740, the preform is consolidated such as in an autoclaveunder heat and pressure. At 750, the preform is subject to a burn-outprocess so that, for example, the preform matrix has a monomodal poresize distribution. At 760, the cured perform is subjected to chemicalvapor infiltration to densify the cured preform. Further, densificationmay be occur at 763 by application of a slurry cast followed at 767 bymelt infiltration to form a finished ceramic matrix composite articlesat 770. The slurry cast may include silicon carbide, silicon nitride,molybdenum silicides, boron carbide, HfC, ZrC, HfB2, ZrB2, rare earthsilicates, and the melt infiltration may include silicon, siliconalloys, silicides, oxide, or combinations thereof. A ceramic matrixcomposite articles formed by method 700 may have a volume porosity ofless than about 5 percent. The ceramic matrix of the ceramic matrixcomposite may have a monomodal pore size distribution with a median poresize of about 1 micrometers to about 20 micrometers. The ceramic matrixof the ceramic matrix composite may have a uniform spacial poredistribution. Such a ceramic matrix composite article may beadvantageous for application to silicon-bearing ceramic turbinecomponents, for example, turbine blades, vanes, nozzles, shrouds,combustors, etc., and repairs thereof.

The further densification in methods 600 and 700 using melt infiltrationmay result in ceramic matrix composite articles that are fully dense,e.g., having generally zero, or less than about 5 or less that about 3percent by volume residual porosity. This very low porosity gives thecomposite desirable mechanical properties, such as a high proportionallimit strength and interlaminar tensile and shear strengths, highthermal conductivity and good oxidation resistance. The matrices mayhave a free silicon phase (i.e. elemental silicon or silicon alloy) thatmay limits the use temperature of the ceramic matrix composite articlesto below that of the melting point of the silicon or silicon alloy, orabout 2,550 degrees Fahrenheit to 2,570 degrees Fahrenheit. The freesilicon phase may result in a lower creep resistance compared todensification solely by chemical vapor infiltration.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Numerous changes and modificationsmay be made herein by one of ordinary skill in the art without departingfrom the general spirit and scope of the disclosure as defined by thefollowing claims and the equivalents thereof. For example, theabove-described embodiments (and/or aspects thereof) may be used incombination with each other. In addition, many modifications may be madeto adapt a particular situation or material to the teachings of thevarious embodiments without departing from their scope. While thedimensions and types of materials described herein are intended todefine the parameters of the various embodiments, they are by no meanslimiting and are merely exemplary. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the various embodiments should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled. In the appendedclaims, the terms “including” and “in which” are used as theplain-English equivalents of the respective terms “comprising” and“wherein.” Moreover, in the following claims, the terms “first,”“second,” and “third,” etc. are used merely as labels, and are notintended to impose numerical requirements on their objects. Also, theterm “operably” in conjunction with terms such as coupled, connected,joined, sealed or the like is used herein to refer to both connectionsresulting from separate, distinct components being directly orindirectly coupled and components being integrally formed (i.e.,one-piece, integral or monolithic). Further, the limitations of thefollowing claims are not written in means-plus-function format and arenot intended to be interpreted based on 35 U.S.C. §112, sixth paragraph,unless and until such claim limitations expressly use the phrase “meansfor” followed by a statement of function void of further structure. Itis to be understood that not necessarily all such objects or advantagesdescribed above may be achieved in accordance with any particularembodiment. Thus, for example, those skilled in the art will recognizethat the systems and techniques described herein may be embodied orcarried out in a manner that achieves or optimizes one advantage orgroup of advantages as taught herein without necessarily achieving otherobjects or advantages as may be taught or suggested herein.

While the disclosure has been described in detail in connection withonly a limited number of embodiments, it should be readily understoodthat the disclosure is not limited to such disclosed embodiments.Rather, the disclosure can be modified to incorporate any number ofvariations, alterations, substitutions or equivalent arrangements notheretofore described, but which are commensurate with the spirit andscope of the disclosure. Additionally, while various embodiments havebeen described, it is to be understood that aspects of the disclosuremay include only some of the described embodiments. Accordingly, thedisclosure is not to be seen as limited by the foregoing description,but is only limited by the scope of the appended claims.

This written description uses examples, including the best mode, andalso to enable any person skilled in the art to practice the disclosure,including making and using any devices or systems and performing anyincorporated methods. The patentable scope of the disclosure is definedby the claims, and may include other examples that occur to thoseskilled in the art. Such other examples are intended to be within thescope of the claims if they have structural elements that do not differfrom the literal language of the claims, or if they include equivalentstructural elements with insubstantial differences from the literallanguage of the claims.

1. A method for forming a ceramic matrix composite article, the methodcomprising: curing a shaped preform comprising a prepreg tape layup ofunidirectional arrays of fiber tows, a matrix precursor, and a porefernier to molyze the matrix precursor and burnout the pore former sothat the shaped preform comprises the unidirectional arrays of fibertows and a porous matrix skeleton having a monomodal pore sizedistribution, wherein a median of the monomodal pore size distributionof the cured preform is between about 1 micrometers and about 30micrometers: and subjecting the cured shaped preform to chemical vaporinfiltration to densify the porous matrix skeleton so that the ceramicmatrix composite article has a fiber volume fraction between about 15percent and about 35 percent.
 2. (canceled)
 3. The method of claim 1wherein the porous matrix skeleton comprises a uniform spacial porositydistribution.
 4. The method of claim 1 wherein the porous matrixskeleton comprises a ceramic.
 5. The method of claim 4 wherein theceramic comprises silicon carbide.
 6. The method of claim 4 wherein theceramic is derived from the pytolysis of the matrix precursor.
 7. Themethod of claim 1 wherein the matrix precursor is polycarbosilanes,tetraethyl orthosilicates, polysiloxanes, phenolics, furanic compoundsand/or polysilazanes.
 8. The method of claim 1 wherein the subjectingcomprises subjecting the cured shaped preform to a gaseous mixture thatdeposits silicon carbide.
 9. The method of claim 5 wherein thesubjecting the cured shaped preform to chemical vapor infiltrationcomprises subjecting the cured shaped preform to a gaseous mixture thatdeposits silicon carbide.
 10. The method of claim 1 wherein the ceramicmatrix composite article comprises interlaminar tensile, strength ofover about 6 ksi.
 11. (canceled)
 12. The method of claim 1 wherein amedian of the monomodal pore size distribution of the cured preform isbetween about 1 micrometers and about 20 micrometers.
 13. The method ofclaim 1 wherein the chemical vapor infiltration comprises a partialchemical vapor infiltration, and further comprising subjecting thepartial chemical vapor infiltration densified ceramic matrix compositearticle to a melt infiltration,
 14. The method of claim 13 wherein themelt infiltration comprises silicon, a silicon alloy, or oxide.
 15. Themethod of claim 13 wherein after the ceramic matrix composite articlesubjected to the melt infiltration it comprises a porosity less thanabout 5 percent.
 16. The method of claim 1 wherein the chemical vaporinfiltration comprises a partial chemical vapor infiltration, andfurther comprising subjecting the partial chemical vapor infiltrationdensified ceramic matrix composite article to a slurry casting and amelt infiltration.
 17. The method of claim 16 wherein the slurry castingcomprises a slurry comprising silicon carbide, boron carbide, one ormore oxides, and/or combinations thereof.
 18. The method of claim 1wherein the cured shaped preform comprises a volume porosity of about 35percent to about 65 percent.
 19. The method of claim 1 wherein theceramic matrix composite article comprises a volume porosity of about 5percent to about 20 percent.
 20. The method of claim 1 wherein theceramic matrix composite article comprises at least one first portionhaving a first fiber volume percentage and at least one second portionhaving a second fiber volume percentage different from said first fibervolume percentage.
 21. The method of claim 1 wherein the pore formercomprises polyethylene, polypropylene, polyamide, nylon,polytetrafluoroethylene, polystyrene, polyvinyl acetate, polyvinylalcohol, or cellulosic powders.
 22. The method of claim 1 wherein theshaped prepreg further comprises silicon carbide particles, boroncarbide particles, oxide particles, and/or combinations thereof.
 23. Themethod of claim 1 wherein the fiber tows comprise silicon carbide fibertows.
 24. A method for forming a ceramic matrix composite article, themethod comprising: curing a shaped preform comprising a prepreg tapelayup of unidirectional arrays of fiber tows, a matrix precursor forforming a ceramic matrix, and a pore former to pyrolyze the matrixprecursor and burnout the pore former so that the shaped preformcomprises the unidirectional arrays of fiber tows and a porous ceramicmatrix skeleton having a monomodal pore size distribution with a medianpore size of between about 1 micrometers and about 30 micrometers; andsubjecting the cured shaped preform to a partial chemical vaporinfiltration and a melt infiltration, or a partial chemical vaporinfiltration, a slurry casting, and a melt infiltration, to densify theporous ceramic matrix skeleton so that the ceramic matrix compositearticle has a fiber volume fraction between about 15 percent and about35 percent. 25.-34. (canceled)
 35. The method of claim 1, wherein eachfiber tow comprises about 500 individual fibers.
 36. The method of claim35, wherein the fibers have a diameter range of about 5 to 20micrometers.
 37. The method of claim 1, wherein the shaped prepregfurther comprises particles or whiskers of SiC, B₄C, SiO₂, HfC, HtB₂,ZrC, ZrB₂, MoSi₂, Si₃N₄, Al₂O₃, rare earth silicates, or rare earthsilicides.
 38. The method of claim 1, wherein the shaped prepreg furthercomprises a carrier comprising water, isopropanol, toluene, or acetone.39. The method of claim 9, wherein the gaseous mixture comprisesmethyl-trichlorosilane, dimethyl-dichlorosilane, silane±methane, and/ortetracholosilane+methane.
 40. The method of claim 16, wherein the slurrycast comprises silicon carbide, silicon nitride, molybdenum silicides,boron carbide, HfC, ZrC, HfB2, ZrB2, or rare earth silicates.
 41. Themethod of claim 40, wherein the melt infiltration comprises silicon, asilicon alloy, a silicide, an oxide, or a combination thereof.
 42. Themethod of claim 21, wherein the pore former comprises particles having amedian size in a range of about 1 micrometer to about 30 micrometers.43. The method of claim 21, wherein the pore former comprises particleshaving a median size in a range of about 1 micrometer to about 20micrometers.
 44. The method of claim 21, wherein the pore formercomprises particles having a median size in a range of about 3micrometers to about 10 micrometers.
 45. The method of claim 21, whereinthe pore former comprises particles having a median size in a range ofabout 3.5 micrometers to about 8 micrometers.